As the variety and number of sensor elements continue to be developed for the automotive, consumer, industrial and medical markets, the plethora of signal drive, acquisition and processing solutions have also likewise increased in direct proportion. Low volume applications are generally limited to custom circuits of discrete active and passive electronic components where the elements of cost, size, power consumption and reliability are poorly optimized. In higher volume applications, where the development of semi-custom or fully custom application specific integrated circuits (ASIC) can be justified from a cost and risk standpoint, the overwhelming practice is to develop specific solutions for specific sensor types and their specific applications. This practice is very inefficient from the standpoint of time-to-market, development cost, risk and part price. The development cycle of even the simplest ASIC generally requires at least one year or more of development time in addition to the investment of millions of dollars. At the end of this development effort, there is no guarantee that the ASIC will work to the required specifications for the particular sensor application.
Sensor ASICs are some of the most difficult designs to realize properly, especially when they are combined with digital logic functionality, as is almost always necessary for the interface requirements into a larger system. Such mixed signal designs are particularly susceptible to crosstalk injected from the digital into the analog sections of the ASIC which severely degrades sensor performance. Having a well characterized IC based sensor platform is highly desirable, as once its characteristics have been quantified, understood and optimized, there is no need to re-invent it for use with other sensors. In addition to the obvious time and cost savings gained through the elimination of another custom ASIC development, the risk of a part not working is almost entirely mitigated. In the world of ASIC designs, modeling and predictive tools are now commonly available to avoid costly mistakes when implementing a new IC. However, such tools are almost exclusively limited to the domain of digital logic. There are very few analog modeling tools that effectively capture analog part performance, especially when mixed with digital logic. This is especially the case as silicon process geometries continue to shrink. Layout, isolation, leakage, and many other unquantifiable electrical interactions stand as critical issues in analog and mixed signal designs and remain more an area of art than science. No modeling tools exist on a system level for mixed signal ASIC design as the high complexities and subtle nuances of analog and digital interactions are impossible to adequately model. Therefore, the risk of failure or suboptimal performance abounds during design implementation.
Therefore, the design cycle of a successful mixed signal sensor ASIC is mostly a matter of successive iterations of trial-and-error, involving a tremendous amount of tedious “grunt work” as each new ASIC “spin” has to be fully characterized and then the problems must be understood and fixed. Often, this becomes a series of very expensive controlled experiments, fixing all parameters in place except for one, and then seeing whether the performance converges or diverges. Add to this, the possibilities of digital glitches occurring, and it seems miraculous that any mixed signal ASIC can ever make it into production at all. As can be inferred from this description, it would be highly advantageous to have a single known working platform that can form the basis for all sensor signal drive, acquisition and processing variants. Once working well, this proverbial “wheel” would not have to be re-invented for subsequent applications, and a more gradual evolution of feature sets can then be more safely and efficiently achieved from such a solid base.
In theory, the universal platform approach makes a lot of sense and would seem to have a great deal of attractiveness for many within the sensor industry. But in reality, until the present invention, this prospect has remained mostly a “Holy Grail”—ideal but unachievable. The problem for the sensor market stems from the fact that for any given physical parameter that is desired to be measured, there exist many different possible sensor element solutions, all with correspondingly different electrical characteristics. When compounded with the fact that there exists sensors that can measure just about every known physical parameter-pressure, acceleration, gas, chemical, magnetism, force, tilt, temperature, light, proximity, rate, torque, humidity, etc.—it becomes readily apparent that having a single signal drive, acquisition and processing platform, however desirable, is not a trivial undertaking. Each of the above sensor categories (in addition to others not enumerated), can have solutions where the sensor element has an electrical change in the form of resistance, capacitance, inductance, voltage, or current. Couple this with the fact that the base impedances can be at extreme ends of a large spectrum and can have signal levels just as diverse, and the challenge of creating a universal sensor platform seems insurmountable.
The sensor market is in desperate need of a standardized platform that can facilitate and simplify the effort necessary to make any sensor element work for a user's intended application. Presently the options for sensor signal extraction are typically limited to Wheatstone bridge configurations where the bridge signal is buffered and amplified by an instrumentation amplifier whose resulting output signal is fed into an analog-to-digital converter. Such a configuration limits the type and breadth of sensor elements that can be driven and operated as the impedances, signal voltage levels, and dynamic ranges between different sensor elements are vastly different. Additionally, it is the case that an inductive sensor needs a much different signal extraction circuit than does a resistive or capacitive sensor element. Clearly, there is a need for the present invention.